Numerous variations of opposed face-type seals, often referred to as mechanical seals, have been developed for use in creating a sealed relationship between a rotatable shaft and a surrounding housing. Such seals, including those of the type which are often referred to as gas seals, are conventionally positioned within a stuffing box or seal chamber which is defined within the housing in surrounding relationship to the shaft. In many types of equipment requiring use of such seals, however, and particularly pumps, the stuffing box or seal chamber is of an extremely small size (that is, the stuffing box is of a small radial clearance as measured between the inner diameter of the surrounding housing and the outer diameter of the rotatable shaft). For example, large numbers of pumps often referred to as ANSI pumps provide only about 5/16 to 3/4 inch radial bore clearance for accommodating the seal or packing, and positioning an effective opposed face-type seal in such small space has generally been extremely difficult, and hence other types of sealing or packing arrangements have often been utilized on pumps having small packing or stuffing box chambers (such pumps often being referred to as having small bore seal chambers). The problem is further complicated by the fact that such pumps, adjacent the end of the stuffing box, also generally provide little available space for mounting a seal exteriorly of the stuffing box.
While numerous mechanical seals of the opposed face type have been developed and utilized on fluid handling equipment including pumps, nevertheless most such seals have been undesirably large, both radially and axially, and hence have not been suitable for adaptation to and use on small bore pumps. For example, numerous double mechanical seals have been developed, including not only contacting face-type, but also gas seals which include both contacting and non-contacting face types. These known seals, however, in addition to exhibiting the conventional largeness which restricts their application to small-bore pumps, have also conventionally possessed the recognized problems of secondary seal hang-up and incompatibility of O-rings with the pumped fluid. In an effort to eliminate or minimize these latter problems, many of the known mechanical seals of both the gas and non-gas types have employed metal bellows to eliminate the O-rings as secondary seals and thus attempt to minimize the secondary seal hang-up and seal ring compatibility problems. The seals employing metal bellows, however, have generally employed rather large complex bellows arrangements which have typically employed multiple bellows convolutions provided with retaining flanges or elements at opposite ends, which retaining flanges in turn are either pressed, fitted or otherwise fixedly secured to the adjacent members of the seal assembly. These bellows have hence significantly increased the structural and manufacturing complexity of the seal assembly, and have resulted in the seal assembly being of significant size, particularly with respect to axial length. Such seal bellows also make it difficult to achieve a predictable and maintainable balance diameter when the bellows is subjected to the pressures of the pump process fluid on one side and a barrier fluid on the other side, and hence this inability to maintain an accurate balance diameter can cause variations in the balance pressure and accordingly can effect the amount of fluid which escapes or passes between the opposed seal faces, particularly in a non-contacting-type gas seal.
The use of conventional bellows in known seals, as discussed above, further complicates the proper application of forces to the seal rings, particularly with respect to the application of these forces relative to the centroids of the seal rings, and thus maintaining proper convergence and hence proper opposed contacting or adjacent relationship between the opposed seal faces is further complicated. The fact that many of the known bellows also have a collar at one end which typically has a shrink fit onto the seal ring also induces additional stresses into the seal ring which can interfere with proper seal ring configuration and hence can affect the proper convergence of the seal faces.
In the conventional bellows-type seals, particularly double seal units which utilize an intermediate chamber containing a barrier fluid for isolating the sealed product from the environment, the problem of providing an effective and compact sealing relationship
is further complicated by the fact that the barrier fluid and product pressures act on opposite sides of the bellows associated with the inboard seal unit, and in those rare situations where a failure of barrier fluid pressure occurs, maintaining an effective seal between the product and barrier chamber is complicated by the excessive unbalanced product pressure which exists on the inboard seal unit and which can cause undesired contact pressures and distortion forces so that maintaining an effective seal under this condition becomes even more difficult to achieve.
Accordingly, it is an object of this invention to provide an improved bellows-type face seal, particularly a double gas seal, having a desirably small and compact configuration so as to be particularly adaptable for use on pumps having small bore seal chambers, which improved seal is believed to overcome many of the disadvantages associated with conventional seals as briefly summarized above.
More specifically, this invention relates to an improved gas seal having inboard and outboard seal units each defined by a pair of opposed and relatively rotatable seal faces, one face of each pair having appropriate grooves which communicate on one seal periphery with a pressurized barrier gas, preferably an inert gas such as nitrogen. The inboard seal unit, at its other seal periphery, communicates with the process fluid being handled by the rotating equipment, namely the pump. At least the inboard seal unit has one of the seal rings, the nonrotating ring in the preferred embodiment, urged axially toward its opposed ring by a compact bellows which axially cooperates between the respective seal ring and a backing member. The bellows is positioned to create an isolation between the barrier and process fluids and to provide a mean effective diameter to provide desirable balance of pressures as imposed on the axially moving seal ring to optimize seal performance.
In the improved seal of this invention, as aforesaid, the bellows is of a minimum number of convolutions, such as about three to five convolutions in the preferred embodiment, each defined by two bellow plates or leaflets which are sealingly engaged along one radial edge with the leaflets of adjacent convolutions being sealingly engaged along the other radial edge. The bellows at one end is joined to a support ring which in turn concentrically supports the stationary seal face. A deformable or elastomeric seal element or gasket is axially and sealingly compressed between the support ring and seal face. This compressible seal ring is preferably disposed so that it is at least slightly outside the mean pressure diameter of the bellows to provide the desired pressure balance on the seal ring. The opposite or outer end of the bellows is joined to a balance piston which is axially slidably supported on the surrounding stationary housing or gland. The balance piston has a rear face exposed to the product pressure, and in addition has a front face which is exposed to the barrier pressure so that the latter normally maintains; the balance piston in a position of abutment with the stationary gland or housing. The balance piston is normally axially spaced from the face ring, and has a face part which is axially aligned with but normally spaced from the support ring. Upon failure or absence of barrier pressure, or if the process pressure improperly exceeds the barrier pressure during an operating excursion, the process pressure acting against the rear face of the balance piston causes the latter to slide axially toward the face ring causing limited compression of the bellows until the face part of the balance piston abuts the support ring, whereby process pressure applied to the balance piston is then transferred onto the support ring and through the compressible seal ring for application to the face ring. All forces applied to the face ring to maintain the seal faces closed, in the event of a barrier fluid pressure failure, are thus transmitted first to the support ring and thence through the compressible seal ring to thus prevent hard face-to-face contact between the components and thus minimize the application of distortion-inducing forces to the face ring.
The improved seal of this invention, due to the structure and functional cooperations created by the bellows, minimizes or eliminates the secondary seal drag problem created when the secondary seal is defined by conventional O-rings, and the bellows is still able to accurately maintain a proper pressure balance diameter. The bellows enables the seal to operate with higher temperature process fluids without encountering problems of seal degradation or incompatibility with the process fluid, and can be constructed free of special end collars or the like and hence can be connected with the adjacent support ring and backing member solely through a fixed ringlike contact to simplify the construction and minimize undesired distortion forces, particularly on the seal ring. The overall seal can be manufactured in a very small and compact size so as to be particularly adaptable for mounting exteriorly of but directly adjacent the stuffing box of a pump, with the overall seal being effectively constructed preferably as a cartridge to facilitate its adaptation to the shaft of the pump.
Other objects and purposes of the invention will be apparent to persons familiar with structures of this general type upon reading the following specification and inspecting the accompanying drawings.